Self-calibrating control methods and systems for refrigeration systems

ABSTRACT

Disclosed are refrigeration system control systems and methods for compressor motor protection and defrost control. The disclosed systems and methods are generic in the sense that they are self-calibrating and so may be employed in a variety of different air conditioner or heat pump models of different sizes and capacities, without being specifically tailored for a particular model. The disclosed systems and methods sense loading on the compressor and evaporator fan motors, preferably by sensing the voltage across the capacitor-run winding of an AC induction motor and normalizing with respect to line voltage. The self-calibrating capability is implemented by taking advantage of the changing loads as a function of time on both the compressor and fan motors during both normal and abnormal operation of a refrigeration system. In overview, a reference value of motor loading is established for each motor at certain times during an ON cycle. At later times the then-prevailing motor loading is compared to the stored reference in order to provide a basis for control decisions. The ratio of capacitor-run winding voltage to line voltage is an advantageous indicator of motor loading. In one embodiment, a reference ratio of capacitor-run winding voltage to line voltage is established, and at later times is compared to the then-prevailing ratio of capacitor-run winding voltage to line voltage.

CROSS-REFERENCE TO RELATED APPLICATION

This is a division of application Ser. No. 778,076 filed Sept. 20, 1985,now U.S. Pat. No. 4,653,285.

BACKGROUND OF THE INVENTION

The present invention relates to control systems and methods forrefrigeration systems, including air conditioners and heat pumps, whichcontrol systems avoid the need for expensive sensors and which arecapable of functioning in a variety of refrigeration system models,without adjustment or selection. In this regard, the control methods andsystems of the present invention may be termed "generic" in that asingle control system is capable of serving a large number of differentmodels, of widely differing capacities.

The present invention is particularly concerned with refrigerationsystems of the type employed in air conditioners and heat pumps forcooling and heating living spaces. Such units are available in a widevariety of physical configurations and capacities, including small roomair conditioners, self-contained reversible heat pump systems whichsomewhat resemble room air conditioners, but which provide both heatingand cooling, central air conditioning systems which employ an indoorevaporator and a separate outdoor compressor/condenser combination, andsimilarly-configured heat pump systems which provide both heating andcooling by means of a reversible refrigeration system.

Such refrigeration systems, while apparently simple to control, in factrequire fairly sophisticated control systems if proper operation andprotection from damage under a wide variety of operating conditions,often adverse, are to be achieved. In addition, both heat pumps and airconditioners require periodic defrosting of the evaporator. For highestefficiency, defrosting should be done only when necessary.

Typical prior art control systems for protecting refrigeration systemsemploy a number of sensors so that the control system is provided withsufficient information upon which to base control decisions. Forexample, a common operating condition to which refrigeration systems aresubjected is so-called "short cycling" which results when an attempt ismade to restart the refrigerant compressor shortly after it has beenrunning and before pressures within the closed circuit refrigerationsystem have had time to equalize. This condition typically resultsfollowing a momentary power interruption, or as a result of useradjustment of a thermostatic control in a manner which causes thecompressor to attempt a restart right after it has stopped. Thecompressor is unable to start under load, and hence stalls. Thus,typical control systems sense the overcurrent condition which resultswhen the compressor motor is stalled, and de-energize the compressormotor for a cooling off period if the over current condition persistsfor more than a few seconds. Thermal overload protectors provide similarresults.

A related adverse condition is simply a high load condition, which canresult when power line voltage is excessively low (a so-called "brownout" condition), or when operating under extreme ambient temperatureconditions. Thus, on an extremely hot day, an air conditioning systemmay be subjected to both a high load and low voltage. This tends to makethe motor inefficient, which leads to over heating. Under such operatingconditions, it is desirable to de-energize the compressor before damageresults, and then allow operation to resume after a cooling-offinterval.

Other compressor protection systems employ pressure sensors connectedinto the high-pressure side of the refrigeration system in order tosense excessive pressures, and de-energize the compressor when theseoccur.

By way of more specific example, various motor and compressor protectionsystems are disclosed in the following U.S. Patents: Anderson et al U.S.Pat. No. 4,038,061; Godfrey U.S. Pat. No. 4,079,432; Newell U.S. Pat.No. 4,253,130; and Genheimer et al U.S. Pat. No. 4,286,303. Of these,Anderson and Newell disclose relatively comprehensive systems forprotecting air conditioners and heat pumps, and employ a variety ofcurrent and temperature sensors. Godfrey and Genheimer et al disclosemotor protection systems in general which include the function ofallowing a motor to attempt a restart following an overload, but onlyfor a limited number of times.

Another approach to motor protection, particularly for a refrigerationsystem compressor motor, is disclosed in commonly-assigned Pohl U.S.Pat. No. 4,196,462. As disclosed in that patent, a single-phase ACinduction motor of the type employing a capacitor-run winding can beprotected from overload and overspeed conditions by monitoring thevoltage across the capacitor-run winding. Under heavy loadingconditions, the winding voltage decreases. This can be sensed, and usedto initiate appropriate protection measures, such as a timed cooling-offinterval. The system described in U.S. Pat. No. 4,196,462 alsoinherently recognizes a locked-rotor condition.

Defrost control systems typically employ from one to three temperaturesensors in order to recognize particular conditions characteristic ofexcessive evaporator frost, and to initiate a defrosting operation whenthis occurs. Examples of such systems are disclosed in commonly-assignedNolan et al U.S. Pat. No. 4,102,391 and commonly-assigned Pohl U.S. Pat.No. 4,215,554.

Another approach to detecting excessive ice formation on an evaporator,particularly the outdoor heat exchanger of a heat pump system, isdisclosed in Gephart et al U.S. Pat. No. 4,123,792. Gephart et alrecognize that ice buildup changes the loading on the evaporator fanmotor. The degree of motor loading is monitored and detected bydeveloping a signal proportional to the average product of motor currentmultiplied by the cosine of the phase angle between motor current andmotor voltage.

In a related approach, Fowler U.S. Pat. No. 4,420,072 discloses a loadindicator for a blower motor which circulates air through an air filter.When the filter becomes dirty, this condition is recognized by a changein motor loading.

From the foregoing brief background, it will be appreciated that priorart control systems not only require a relatively large number ofdiverse sensors, but also must be particularly adjusted to the size ofthe unit involved. Thus, overcurrent protection sized for a small airconditioner would be entirely inappropriate for a large one. By way ofexample, a typical product line may have from twenty to thirty differentmodels, each requiring a customized control system.

SUMMARY OF THE INVENTION

It is an object of the invention to provide refrigeration system controlsystems and methods which are generic in the sense that they areself-calibrating and so may be employed in a variety of different airconditioner or heat pump models without being specifically tailored fora particular model, or even a particular individual unit.

It is another object of the invention to provide such control systemsand methods which avoid the need for a variety of specialized sensors.

It is another object of the invention to provide such systems andmethods which are applicable to compressor motor protection as well asto defrost control.

In accordance with one overall aspect of the invention, it is recognizedthat sensed motor loading, however sensed, may advantageously beemployed for a variety of protection and control purposes by controlsystems which are self-calibrating. In accordance with the invention,sensing motor loading at various times provides sufficient informationon which to base control decisions for a number of functions including,but not limited to, overload protection, defrosting and protectionagainst short cycling.

Motor RPM is a convenient indicator of motor loading, and an excellentindicator of motor stress. Rather than directly sensing motor RPM, it isrecognized that a form of the sensing system of the above-identifiedcommonly-assigned Pohl U.S. Pat. No. 4,196,462 can be employed to greatadvantage. More particularly, where a refrigeration system employssingle-phase AC induction motors of the type including a "run" windingand a separate capacitor-run winding to provide a "split" phase, duringoperation of such a motor the ratio of voltage across the capacitor-runwinding to line voltage provides a sensitive indicator of motoroperating conditions.

It will be appreciated that, in accordance with the broader aspects ofthe invention, other techniques for sensing motor loading may beemployed. By way of example and not limitation, the motor loadingsensing techniques of Gephart et al U.S. Pat. No. 4,123,782 and FowlerU.S. Pat. No. 4,240,072 can each be adapted to the self-calibratingcontrol and protection techniques of the present invention.

A particularly significant aspect of the invention is itsself-calibrating capability which takes advantage of the changing natureas a function of time of the load on both the compressor and fan motorsduring both normal and abnormal operation of a refrigeration system.Thus, in overview, in accordance with the invention, reference values ofmotor loading (in an exemplary embodiment as reflected by the ratio ofcapacitor-run winding voltage to line voltaqe) are established atcertain times during an ON cycle. At later times the then-prevailingmotor loadings (or the then-prevailing ratios of capacitor-run windingvoltage to line voltage) are compared to the stored references in orderto provide a basis for control decisions.

In general, in refrigeration systems, adverse conditions such ascompressor overheating or excessive frost develop only after aconsiderable time has elapsed from the beginning of a run. For example,frost takes at least an hour to build to an undesirable level, causingexcessive pressures and temperatures in the system. Motor temperaturesrise slowly, causing increased copper losses and adverse stressesreflected in performance output

More particularly, in accordance with the invention, protection for amotor driving a refrigerant compressor in a refrigeration system whichis cycled ON and OFF during operation is provided as follows. Followingthe start of a compressor ON cycle, a stabilization interval is allowedto elapse during which start-up transients, liquid slugging effects, andthe like have dissipated, but before the compressor is significantlyloaded as a result of pressure build up. The duration of thestabilization interval is in the range of five seconds to five minutes.A typical stabilization interval is thirty seconds. After thestabilization interval has elapsed, a compressor motor reference loadingis determined by sensing the motor loading at that particular time andstoring it as a reference loading. In the preferred form of theinvention where voltage ratios are sensed the ratio of capacitor-runwinding to line voltage is sensed after the stabilization interval haselapsed, and stored as a compressor motor reference ratio. This approachis effective because the ratio of capacitor-run winding to line voltageis largely independent of normal line voltage fluctuations.

Thereafter, during each compressor ON cycle, motor loading is at leastperiodically sensed, and compared to the stored reference. If thethen-prevailing loading has increased above a high load thresholdloading established as a predetermined function of the referenceloading, then overloading is indicated, and the compressor isde-energized. In the preferred form of the invention, it is thethen-prevailing ratio of capacitor-run winding voltage to line voltagewhich is sensed and compared to the compressor motor reference ratio.The compressor motor is de-energized if the then-prevailing ratio fallsbelow a high-load threshold ratio established as a predeterminedfraction of the reference ratio. The high-load threshold ratio ispreferably on the order of 0.75-0.80 times the reference ratio.

Significantly, this approach can be made self-calibrating, andcompressor motor protection is afforded regardless of the size of themotor, since the motor control system of the invention establishes itsown reference based on the characteristics of the particular motor.Moreover, since the voltage-sensing technique of the above-incorporatedPohl U.S. Pat. No. 4,196,462 inherently measures instantaneous motorstress rather than actual heat build-up, the motor can be protectedbefore a temperature rise actually takes place. Thus, a sudden change inload produced by blockage, for example, can be detected, and the motorprotected before excessive temperatures are reached. Similarly, if anexcessive drop in line voltage occurs with a consequent drop in RPM anda corresponding increase in lost power due to motor inefficiency, thesubject control system will anticipate excessive temperatures to provideenhanced motor protection.

In the event the compressor motor has been de-energized as a result ofthe then-prevailing ratio falling below the high-load threshold ratio,the motor is re-energized after a cooling-off time interval has elapsed.A cooling-off time interval of in the order of ten minutes is typical.

Another form of compressor protection afforded quite advantageously bythe present invention is a locked rotor condition caused by thecompressor failing to start at all, or stopping during operation due toan extreme overload. In accordance with this aspect of the invention, atthe beginning of a compressor ON cycle, a compressor motor equilibriumspeed interval, for example two seconds is allowed to elapse, and thethen-prevailing ratio of capacitor-run winding voltage to line voltageis sensed. If this ratio is below a predetermined locked rotor ratio,then a locked rotor condition is recognized, and the compressor motor isde-energized. It turns out that the use of one locked rotor ratio iseffective for virtually all motors regardless of their rated size. Anexemplary locked rotor ratio is 0.5. Thus, if after two seconds, themotor's capacitor-run winding voltage to line voltage is less than 0.5,the locked rotor condition is recognized.

When the locked-rotor condition is recognizes, the compressor isde-energized for a cool-down interval of typically 2.5 minutes.Thereafter, the compressor motor is re-energized for a restart attempt.Normally, this condition occurs under "short cycling" conditions, andeventually the refrigeration system pressures approach equilibrium,removing the load from the compressor, which then starts. For the eventsome other problem is causing the locked rotor condition, the number ofrestarts is counted and limited to a predetermined number, for examplenine, after which the system shuts down entirely.

In accordance with the invention, related self-calibrating techniquesare applied to controlling defrosting operations. For most defrostcontrol purposes, loading on the evaporator fan motor is sensed,preferably by sensing the ratio of capacitor-run winding voltage to linevoltage. For some defrosting conditions, it is also advantageous tomonitor the compressor motor since, in the presence of excessiveevapovator frost buildup, the load on the compressor motor is reduced.

More particularly, in accordance with the invention, defrosting of anevaporator in a refrigeration system is controlled by the followingself-calibrating technique. When a refrigeration system ON cycle begins,an airflow stabilization interval, on the order of ten seconds isallowed to elapse. During the airflow stabilization interval, evaporatorairflow stabilizes at a rate corresponding to an unblocked evaporator.At that time, a fan motor reference loading is determined and stored asa reference. In the preferred embodiments, the ratio of fan motorcapacitor-run winding voltage to line voltage is employed as anindicator of fan motor loading. Thus, a fan motor reference ratio isdetermined by sensing and then storing the ratio of capacitor-runwinding voltage to line voltage. As evaporator frost builds up andincreasingly blocks airflow, the load on the fan motor decreases becausethe fan moves less air and therefore does less work. Thus, thereafter,during each ON cycle, prevailing fan motor loading is sensed at leastperiodically, and compared to the reference loading. A defrostingoperation is initiated if sensed loading is less than a low-loadthreshold loading established as a predetermined function of thereference speed.

In the preferred embodiments of the invention where capacitor-runwinding voltage is sensed, the then-prevailing ratio is compared to thereference ratio, and a defrosting operation is initiated if theprevailing ratio exceeds a low-load threshold ratio established as apredetermined fraction in excess of the reference ratio. A low-loadthreshold ratio of approximately 1.08 times the fan motor referenceratio has been found to be suitable. This will depend to some extent onthe type of fan blades used; the figure given is particularly applicableto blower-type air movers.

The precise manner of effecting defrosting depends upon the particularsystem, and whether it is an air conditioner or a heat pump. Forexample, an indoor evaporator for an air conditioner can be defrosted bysimply de-energizing the refrigeration compressor while allowing theevaporator fan motor to continue to run. In the case of a heat pump inthe heating mode when the outside coil is functioning as the evaporator,defrosting can be effected by turning off both the compressor and fanmotors. Even at outdoor ambient temperatures down to 25° F., sufficientheat from the refrigeration system normally reaches the evaporator tocause melting of the frost, without the necessity for applying externalheat. Other heat pump systems employ reverse cycle defrost wherein thesystem is in effect placed in a cooling mode to supply heat to theoutdoor coil.

In accordance with another aspect of the invention, the conclusion ofdefrosing of the outdoor evaporator coil of the heat pump is sensed byat least periodically energizing the evaporator fan and checkingevaporator fan loading during speed defrosting operation. When the fanspeed approaches the reference loading, this indicates that theevaporator is again substantially unblocked. This control function ispreferably implemented by sensing the then-prevailing ratio ofcapacitor-run winding voltage to line voltage and comparing the sensedratio to the previously-established reference ratio.

In a manner similar to that summarized above for recognizing a lockedrotor condition of the compressor motor, failure of the fan motor tostart can be detected. For example, failure to start can be caused by amechanical obstruction blocking the fan blades. In accordance with thisaspect of the invention, a fan motor equilibrium speed interval isallowed to elapse. The fan motor equilibrium speed interval is generallywithin the range of two seconds to ten seconds, and is typically threeseconds. At that point, the ratio of capacitor-run winding voltage toline voltage is sensed, and the fan motor is deenergized if this ratiofalls below a predetermined locked rotor ratio. As in the case of lockedcompressor motor protection, a suitable locked rotor ratio is 0.5. Inthe case of a failure of the fan motor to start, no further restarts areattempted, and the compressor is de-energized also to avoid overheatingthe system

In accordance with another aspect of the invention, it is recognizedthat under some conditions the entire system may inadvertently be resetduring a defrosting operation. Such can occur as a result of a momentarypower interruption, or as a result of the user turning the unit OFF thenON while a defrosting operation coincidentally happens to be inprogress. In accordance with the invention, this condition is sensed bya failure of compressor load to build-up after a compressor loadinginterval in the order of ten minutes. Thus, in accordance with theinvention a compressor loading interval is allowed to elapse, andcompressor motor loading is then sensed. If compressor motor loading isbelow a normal load threshold loading established as a predeterminedfunction of the reference speed, then a precautionary defrostingoperation is initiated.

In the preferred forms of the invention where voltage sensing isemployed, a precautionary defrosting operation is initiated if the ratioof capacitor-run winding voltage to line voltage is above a normal loadthreshold ratio established as a predetermined fraction of the refenceratio. Typically, the predetermined fraction is approximately 0.95 timesthe reference ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

While the novel features of the invention are set forth withparticularly in the appended claims, the invention, both as toorganization and content, will be better understood and appreciated,along with other objects and features thereof, from the followingdetailed description taken in conjunction with the drawings, in which:

FIG. 1 is a diagrammatic view of a closed-circuit refrigeration systemsuch as may be employed in a room air conditioner;

FIG. 2 is a similar diagrammatic view of a closed circuit refrigerationsystem employed in a reversible heat pump for effecting both heating andcooling;

FIG. 3 is an electrical schematic diagram depicting one form of controlsystem applied to the refrigeration system of FIG. 1;

FIG. 4 is a similar electrical schematic diagram showing a controlsystem applied to the refrigeration system of FIG. 2;

FIG. 5 is a typical plot of the ratio of capacitor-run winding voltageto line voltage as a function of compressor motor RPM;

FIG. 6 is a typical plot of motor heating (lost power) as a function ofmotor RPM;

FIG. 7 is an exemplary program flow chart depicting an algorithm inaccordance with the invention for compressor protection;

FIG. 8 is a program flow chart in accordance with the inventiondepicting an algorithm for controlling defrosting and for protecting thesystem against a blocked evaporator fan motor;

FIG. 9 is a similar flow chart, depicting the manner in which theconclusion of a defrosting operation is detected, particularly theoutdoor fan of a heat pump; and

FIG. 10 is a program flow chart depicting the manner in which protectionis provided against the possibility that the refrigeration system isreset during a defrosting operation before the defrosting operation isconcluded.

DETAILED DESCRIPTION

Referring first to FIG. 1, shown in highly schematic form is arepresentative closed circuit refrigeration system 10, typical of a roomair conditioner. The system is divided into an indoor side 12 and anoutdoor side 14 by a partition 15. The refrigeration system 10 includesan outdoor condenser 16, an indoor evaporator 18, and a refrigerantcompressor 20 for circulating refrigerant through the system. Althoughnot illustrated, it will be appreciated that the refrigeration system 10also requires a suitable flow restricting or expansion device somewherein the line 22 between the condenser 16 and the evaporator 18, such as acapillary tube or an expansion valve.

The refrigerant compressor 20 is driven by a single-phase AC inductionmotor 24 via a shaft represented at 26. In nearly all cases, thecompressor 20 and the motor 24 are included within a hermetically sealedenclosure. The compressor motor 24 has a pair of AC power inputterminals 28 and 30 supplied from AC power lines L₁ and L₂ via acontrolled switching element 32. As schematically depicted, the motor 24is of the type including a run winding 34 connected directly to theterminals 28 and 30, and a split phase capacitor-run winding 36connected permanently in series with a capacitor 38 across the terminals28 and 30.

While any suitable technique may be employed for sensing motor loading,the presently-preferred technique is to sense voltage across thecapacitor-run winding 36 at a terminal 40.

Air circulation over the condenser 16 and evaporator 18 is respectivelyprovided by a pair of fan blades 42 and 44 driven by a common fan motor46, which is also an AC induction motor of the type having a run winding48 and a capacitor-run winding 50 in series with a capacitor 52. Themotor 46 has terminals 54 and 56 to which AC power is supplied from L₁and L₂ via a controlled switching element 58. Voltage across thecapacitor-run winding 50 is sensed at terminal 60.

The final element depicted in FIG. 1 is a control system 62. The controlsystem 62, via lines 64 and 66, senses the voltage across thecapacitor-run winding 36 of the compressor motor 24, and the voltageacross the capacitor-run winding 50 of the fan motor 46, respectively.The control system 62 also controls the controlled switching elements 32and 58 for energizing the motors 24 and 46 via respective control pathsrepresented at 68 and 70.

In addition to the various motor protection and defrost control aspectsto which the present invention is directed, it will be appreciated thatthe control system 62 in addition typically effects thermostatic controlby cycling the entire system ON and OFF as required. For this purpose,it will be appreciated that the control system 62 also includes at leastone temperature sensing element (not shown), and a means for usertemperature set point adjustment.

During operation, high pressure refrigerant gas from the compressor 20is directed into the condenser 16, and therein condensed by aircirculated past the condenser 16 by the fan 42. Liquid refrigerant thenflows from the condenser 16 to the evaporator 18 via the line 22,including the suitable flow-restricting expansion device (not shown).Within the evaporator 18, liquid refrigerant vaporizes to produce acooling effect, and then returns to the compressor 20. Evaporator fan 44circulates room air past the evaporator 18. Under some circumstances,frost builds on the evaporator 18.

With reference now to FIG. 2, depicted similarly is a reversible closedcircuit refrigeration system 110 employed in a heat pump system for bothheating and cooling. The system is divided into an indoor side 112 andan outdoor side 114 by a representative partition 116.

The reverse cycle refrigeration system 110 includes a refrigerantcompressor 118, an outdoor heat exchanger 120, and an indoor heatexchanger 122. A suitable flow restricting device (not shown) isincluded within the line 124 connecting the outdoor and indoor heatexchangers 120 and 122.

The compressor 118 is connected to the heat exchangers 120 and 122 via areversing valve 126. Thus, each of the heat exchangers 120 and 122 canfunction either as an evaporator or a condenser depending on whetherheating mode or cooling mode operation is desired.

Within the same hermetic enclosure as the compressor 118 is an ACinduction motor 128 having power input terminals 130 and 132 suppliedfrom AC power lines L₁ and L₂ via a controlled switching element 133.The compressor 118 is driven by the motor 128 via a shaft 134. Thecompressor motor 128 has a run winding 136 and a capacitor-run winding138 in series with a capacitor 140. Voltage across the capacitor-runwinding 138 is sensed at a terminal 141.

One or more auxiliary electrical resistance heaters such asrepresentative heater 142 are provided for use when outdoor ambienttemperatures are too low for efficient heat pump operation, or whenauxiliary heat is needed during a heat pump defrosting operation. Therepresentative auxiliary electrical resistance heater 142 is connectedto power lines L₁ and L₂ via a controlled switching element 144.

The compressor motor 128 and the auxiliary heater 142 are selectivelyenergized from the AC line conductors L₁ and L₂ via the respectivecontrol switching elements 133 and 144.

Unlike the simpler system of FIG. 1, the heat pump system of FIG. 2includes separate indoor 147 and outdoor 148 fans driven by respectivemotors 150 and 152. The motors 150 and 152 are also AC induction motorsof the type having respective run windings 154 and 156, respectivecapacitor-run windings 158 and 160 in series with respective capacitors162 and 164. The motors 150 and 152 are energized from AC line L₁ viarespective controlled switching elements 166 and 168, and includeterminals 170 and 172 for sensing of the voltage across thecapacitor-run windings 158 and 160, respectively.

The system of FIG. 2 also includes a suitable control system 174 whichsenses the voltages across the capacitor-run windings 138, 158 and 160,and appropriately operates the controlled switching elements 133, 144,166 and 168.

During operation of the system 110 of FIG. 2 for heating, the reversingvalve 126 directs the flow of high temperature refrigerant gas from thecompressor 118 into the indoor heat exchanger 122 which then functionsas a condenser to warm the air to be conditioned, and to condense therefrigerant gas into liquid form. Indoor air is circulated over the heatexchanger 122 by the fan 147. Refrigerant flows through the line 124,including the expansion device (not shown), to the outdoor heatexchanger 120 which functions as an evaporator, and hence back throughthe reversing valve into the compressor 128. Outdoor air is circulatedover the heat exchanger 120 via the fan 148 During heating modeoperation, the outside heat exchanger 128 functioning as an evaporatoris susceptible to frost build up, restricting the rate of airflow thereacross.

During operation of the system 110 in the cooling mode, refrigerant isdirected via the reversing valve 126 in the opposite direction throughthe heat exchangers 120 and 122. Thus, the outdoor heat exchanger 120functions as a condenser, and the indoor heat exchanger 122 functions asan evaporator, comparable to the operation of the system of FIG. 1.

FIG. 3 depicts a suitable control system, generally designated 62,applied to the refrigeration system of FIG. 1. The FIG. 3 control systemis microprocessor-based, and thus includes a 15 suitable microprocessorcomprising a single-chip microcomputer or microcontroller 200 operatingunder stored program control in a manner well known to those skilled inthe art. While a variety of microprocessor systems may be employed, onewhich is suitable is a Motorola Semiconductor Type No. M6805 Single-ChipN-Channel Microcontroller which includes, within a single integratedcircuit device, program ROM, RAM, a CPU and a variety of I/O linedrivers. In FIG. 3, the controlled switching element 32 of FIG. 1 moreparticularly may be seen to comprise a relay having contacts 202 and acoil 204 driven by a switching transistor 206 in turn driven by anoutput line 208 from the microcontroller 200. Similarly, the FIG. 1switching element 58 for the relatively lower-current fan motor 46 inFIG. 3 more particularly may be seen to comprise a triac 209 drivendirectly by another output line 210 of the microcontroller 200. Thus,the microcontroller 200 can selectively control both the compressormotor 24 and the fan motor 46.

For input sensing, connected to the micro-controller 200 is aninput-multiplexed analog-to-digital (A/D) converter 212. For presentinginputs to the A/D converter 212, three conditioning circuits 214, 216and 218 are included, each comprising a voltage divider for scalingsensed voltage to a lower level, a rectifier and a filter capacitor.More particularly, the conditioning circuit 214 comprises voltagedivider resistors 220 and 222, diode 224 and capacitor 226; theconditioning circuit 216 comprises voltage divider resistors 228 and230, diode 232 and capacitor 234; and the conditioning circuit 218comprises voltage divider resistors 236 and 238, diode 240 and capacitor242.

During operation, each of the conditioning circuits 214, 216 and 218serves to sample with reference to L₂ voltage at the correspondingcircuit node L₁, 40 or 60, recify the voltage, and store it as arespective representative voltage sample V_(L), V_(C) or V_(F) acrossrespective capacitor 226, 234 or 242. The three voltage samples V_(L),V_(C) and V_(F) are respectively for the AC line voltage, the compressormotor 24 capacitor-run winding 36 voltage, and the fan motor 46capacitor-run winding 50 voltage.

It will be appreciated that equivalent results may be achieved byvarious other circuit arrangements.

The circuit time constants are such that the capacitors 226, 234 and 242hold the DC voltage samples for a time consistent with the samplinginterval of the A/D converter 212 and microcontroller 200, which istypically 100 ms. A time constant in the order of 0.5 second is typical.

Any suitable A/D converter 212 can be employed. The resolution should beat least 2% over the range of voltages expected during operation.

FIG. 4 is a similar electrical schematic diagram showing a single-chipmicrocontroller 300 and an analog-to-digital converter 302 similarlyconnected for operating the three motor of the FIG. 2 refrigerationsystem, and sensing the line voltage between L₁ and L₂, as well as thevoltages across the respective capacitor-run windings 138, 158 and 160.Since an additional motor is included, in FIG. 4 there is an additionalconditioning circuit 244, comprising voltage divider resistors 246 and248, diode 250 and capacitor 252.

The conditioning circuits of FIG. 4 operate just like those of FIG. 3,except for the inclusion of an additional motor. Thus in FIG. 4conditioning circuit 214 provides a voltage sample V_(L) representativeof AC line voltage, conditioning circuit 216 provides a voltage sampleV_(C) representative of voltage across the capacitor-run winding 138 ofthe compressor motor 128, conditioning circuit 218 provides a voltagesample V_(Fi) representative of voltage across the capacitor-run winding158 of the indoor fan motor 150, and conditioning circuit 244 provides avoltage sample V_(Fo) representative of voltage across the capacitor-runwinding 160 of the outdoor fan motor 152.

In both FIG. 3 and FIG. 4, it will be appreciated that themicrocontroller 200 or 300 is thus provided with inputs which receivevoltages representing the voltages across the capacitor-run windings ofeach of the AC induction motors in the system, is provided with an inputwhich receives a voltage representative of voltage across the AC lineterminals L₁ and L₂, and is provided with control outputs forcontrolling energization of the various motors.

As noted above, the microcontrollers 200 and 300 operate understored-program control to effect the required decisions to operate thevarious motors. While the details of the programming will depend uponthe specific microprocessors employed, it will be appreciated that thenecessary programming can be represented in high-level flow chart form.Such flow charts are presented in the accompanying FIGS. 8-11, anddescribed hereinbelow.

It is believed that the principles of the invention will be betterunderstood in view of a brief summary of certain characteristics ofsingle-phase AC induction motors with reference to the plots of theaccompanying FIGS. 5, 6 and 7.

Referring now in particular to FIG. 5, depicted is a typical plot 340 ofV_(C) /V_(L) (compressor capacitor-run winding voltage V_(C) normalizedwith respect to line voltage V_(L)) as a function of motor RPM for atwo-pole AC induction motor having a synchronous speed of 3600 RPM. Forsuch motors, useful motor performance is in a relatively narrow band 350between approximately 3500 RPM (light load) and 3200 (heavy load). Ifthe loading on the motor is increased beyond a certain level, the motor"stalls", consistently at approximately 2900 RPM. This figure applies atboth high and low line voltage.

Significantly, the normalized voltage ratio V_(C) /V_(L) provides areliable and sensitive measure of motor RPM and thus motor loading forany practical range of line voltages. Moreover, as is discussed nextbelow with reference to FIG. 6, there is a close correlation betweenmotor heating and RPM, and between the V_(C) /V_(L) ratio and RPM.

More particularly, FIG. 6 includes two typical plots 400 and 402 ofmotor heating (i.e. lost power) in watts as a function of motor RPM fortwo different power line voltages, but at an assumed constant outdoorambient. Plot 400 is for a relatively low line voltage, 208 volts, whileplot 402 is for a relatively high line voltage, 230 volts. A plot 404,comparable to the plot of FIG. 5 but in expanded form shows the shape ofthe V_(C) /V_(L) curve (representing compressor motor loading) over therange of RPMs depicted in FIG. 6. Also shown in FIG. 6 are fourconstant-torque lines 406, 408, 410 and 412 which are included toillustrate what happens when a motor running steadily at a givenconstant torque suffers a change in line voltage.

These constant-torque lines 406, 408, 410 and 412 were experimentallyobtained by measuring RPM and capacitor-run winding voltages employing acompressor calorimeter, with pre-set suction and discharge pressures, atloads which correspond to the outdoor ambient temperatures in a typicalrefrigeration system employed for air conditioning. Line 406 is for apressure differential of 352 psi, simulating an outdoor ambient of 125°F.; line 408 is for a pressure differential of 310 psi, simulating anoutdoor ambient of 114° F.; line 410 is for a pressure differential of268 psi, simulating an outdoor ambient of 108° F. and line 412 is for apressure differential of 223 psi, simulating an outdoor ambient of 95°F.

The constant-torque line 412 (95° F. outdoor ambient) corresponds to thenormal rated BTU of a typical unit, and falls closely on the full loadRPM rating point of the corresponding published motor curve (notillustrated). The highest load likely to be seen in service isrepresented by the constant-torque line 406, which is for a 125° F.outdoor ambient.

In either case, it will be seen that a lower line voltage immediatelyresults in a lower RPM, plus a slight increase in motor heating. Theworst case is clearly at high outdoor ambient temperatures, and low linevoltage.

As noted hereinabove, a refrigeration system characteristic exploited bythe present invention is that the development of heavy loads on thecompressor motor requires a substantial period of time to develop afterinitial startup, typically many minutes. The reason for this is that ittakes several minutes to build up the high load pressures in the systemacross the capillary or expansion valve. The time is a direct result ofthe volume of the system and the restriction afforded by the capillaryor expansion valve, as the case may be.

Typically, about thirty seconds after startup, the V_(C) /V_(L) ratiogently peaks, representing the end of an initial stabilization intervalafter which start-up transients, including liquid slugging effects andthe like have dissipated, but the compressor is not yet significantlyloaded by pressure build up. This peak in the V_(C) /V_(L) ratio may beviewed as a condition of temporary stability where there is a relativelylight load on the motor, while pressures in the system are slowlybuilding up. From the point of view of compressor motor loading or motorspeed, this condition of temporary stability allows a reference to beestablished.

It will be appreciated that the thirty-second figure after which thecondition of temporary stability exists is an exemplary one whichapplies to a particular selection of different models for which it isdesired to provide a "generic" control having self-calibratingcapability. Accordingly, the thirty-second figure may requiremodification for another selection of unit models. In practice, however,the end of the stabilization interval (the beginning of the period oftemporary stability) can be established non-critically within the rangeof five seconds to five minutes. The most important consideration isthat the compressor motor has ceased to accelerate and is still lightlyloaded. Another consideration is that other start-up transients havedissipated.

Compressor motor loading after an initial thirty seconds corresponds inFIG. 6 approximately to a point 422 on the motor loading line 404 wherecompressor motor speed is 3500 rpm. The point 422 is well to the rightof the point 424 on the FIG. 6 motor loading line 404 where thecompressor is operating at its rated load (heavily loaded) and motorspeed is 3200 RPM.

In accordance with invention, the V_(C) /V_(L) ratio existing at thetime of temporary stability after an initial 30 second stabilizationinterval is employed as a reference ratio. Based on empirical testing ahigh load threshold ratio on the order of 0.75-0.8 times the referenceratio has been found to provide suitable results. The low side of thisrange has been found to be more suitable for use with relatively lowefficiency motors with the higer side of the range more suitable forrelatively high efficiency motors. Referring again to FIG. 6, the V_(C)/V_(L) ratio corresponding to point 422 on the curve is 1.25. Applying afactor of 0.8 to this ratio results in a high load threshold ratio of1.0. With this particular high load threshold ratio, the high loadprotection trips at a motor speed on the order of 3100 rpm. Maximumdissipated power is on the order of 800 watts under the worst casecondition of low line voltage and high outdoor ambient temperature. Thisis a substantial improvement compared to the normal capability of athermal overload, which has the fundamental disadvantage that the trippoint under high load conditions must be compromised so that adequatelocked rotor protection is obtained under low line voltage conditions.

Referring now to FIG. 7, shown is a typical program flow chartimplemented in either the microcontroller 200 of FIG. 3 or themicrocontroller 300 of FIG. 4 for providing compressor motor protectionin accordance with the invention. At the outset, it may be noted thatone of the operations called for by the FIG. 7 flow chart is thesampling of the ratio V_(C) /V_(L). It will be appreciated that thisoperation implies separately sampling, via the A/D converter, both thecapacitor-run winding voltage and the then-existing line voltage acrossL₁ and L₂, and performing the necessary division within the CPU of themicrocontroller 200 or 300.

At the outset, it may be noted that the routine of FIG. 7, as well asthe routines of FIGS. 8-10, is merely one part of an overall controlprogram which continuously cycles through each of a number ofsubroutines, including those of FIGS. 7-10. The overall cycle may occurmany times per second such that, in view of the relative slowness of thecontrol events involved in a refrigeration system, from the point ofview of each subroutine, each subroutine is essentially continuouslyexecuted from its entry point. Thus, while waiting for a particular timeinterval to elapse, for example, a particular routine is existed if theinterval has not yet elapsed. However, the routine is re-entered perhapsonly a fraction of a second later. The effect from the point of view ofthat particular routine is equivalent to a wait loop involving thatroutine alone.

For purposes of FIG. 7, V_(L) corresponds to AC line voltage, and V_(C)corresponds to voltage across the capacitor-run winding of the motordriving the compressor.

Considering FIG. 7 in detail, a compressor check routine begins at 500,which is entered when a compressor ON cycle has been initiated by thethermostatic control system (not shown) having called for compressoroperation to effect cooling, in this example. A time variable T isassumed to be initialized to zero. In order to allow the compressormotor locked rotor interval to elapse, program flow enters a delayinterval represented by decision box 502 which exits to 504 each timethrough the routine until such time as the time variable T exceeds twoseconds.

When the compressor motor locked rotor interval has elapsed, the answerin decision box 502 is "yes", and the V_(C) /V_(L) ratio is sampled at506. In order to ensure that the compressor motor has in fact started,the V_(C) /V_(L) is tested in decision box 508 against the locked rotorratio established, for example, as 0.5. From the discussion hereinabovewith reference to FIG. 5, it will be appreciated that the locked motorratio of 0.5 is somewhat arbitrary, inasmuch as effective protectionwould be provided over a relatively large range, for example a lockedrotor ratio range of 0.2 to 0.7.

If a locked-rotor condition exists, then the decision in box 508 is"yes", and program flow branches to box 510 where an appropriate commandis generated to de-energize the compressor by opening an appropriaterelay. The boxes following box 510 implement the dual functions ofestablishing a compressor motor cool-down interval of approximately 2.5minutes prior to another restart attempt, and of limiting the totalnumber of restart attempts to nine. Rather than nine, any appropriatenumber for the restart attempt limit can be established. Typically, thisnumber will be in the range of five to twenty.

Thus, in box 512 a restart counter (assumed initialized to zero) isincremented, and then, in box 514, is tested against the constant nine,which represents the maximum number of restart attempts permitted.

If the decision in box 514 is "no", indicating the maximum number ofrestart attempts has not been exceeded, then a 2.5 minute delay isimplemented in box 516. Thereafter, the compressor is re-energized, andthe time variable T is reset to zero in box 518. The program then loopsback to box 502 at the beginning of another two-second compressor motorequilibrium speed interval.

If the decision in box 514 is "yes", indicating that there have beennine unsuccessful restart attempts, a loop is entered at decision box520 which endlessly loops back via flow chart line 522 until a resetswitch (not shown) is actuated. When the reset switch is actuated, therestart counter is reinitialized to zero in box 524 so that the entiresequence can be performed again, presumably by a service technician.

Returning to decision box 508, assuming the motor has started normallyand thus the ratio V_(C) /V_(L) is greater than the exemplary lockedrotor ratio of 0.5, then the answer in decision box 508 is "no".Following a successful compressor start, a stabilization interval isallowed to elapse which, depending upon the particular system may beanywhere from five seconds to five minutes for reasons explainedhereinabove with reference to FIG. 7. To implement this, executionproceeds to decision box 526 which establishes the stabilizationinterval, in this example thirty seconds. If the stabilization intervalhas not yet elapsed, then the answer in decision box 526 is "yes", andthe V_(C) /V_(L) ratio sampled in box 506 is stored as referencevariable R_(C0). During each pass through the compressor check routine500 during the thirty-second stabilization interval, the referencevariable R_(C0) is thus updated. The final update occurs at T=30 and thevalue of V_(C) /V_(L) at T=30 becomes the final value for the referenceR_(C0) used thereafter.

Upon subsequent passes through the compressor check routine of FIG. 7,the answer in decision box 526 is "no" because the stabilizationinterval is over. Program execution enters decision box 532 wherein thethen-prevailing ratio V_(C) /V_(L) is compared to a high load thresholdratio established as 0.8 times the reference ratio R_(C0).

In the event this test fails, indicating a excessive load on thecompressor motor, the answer in decision box 532 is "yes" and programflow proceeds to box 534 where the compressor is de-energized by openingthe appropriate compressor relay. Box 536 establishes a five-minutecompressor motor cool-down interval, followed by a reset.

In view of the discussion hereinabove of the motor characteristics, itwill be appreciated that the protection technique described effectivelyprotects against motor overloads caused by low line voltages, highoutdoor ambient temperatures, and locked rotor conditions. Moreover, itwill be appreciated that the protection provided is self-calibrating andgeneric in the sense that the same protection control system will serve,without adjustment, a wide variety of different motor sizes.

Referring now to FIG. 8, FIG. 8 is a similar flow chart depicting themanner in which a motor driving an outdoor fan of a heat pump in heatingmode is protected against overheating, and also provided with automaticdefrost control. It will be appreciated by those skilled in the art thatthe microcontroller 200 or 300 executes the flow charts of both FIG. 7and FIG. 8 one after the other at sufficient speeds such that they maybe considered to be simultaneous.

In FIG. 8, V_(L) again corresponds to line voltage, while V_(F)corresponds to voltage across the capacitor-run winding 160 of the motor152 driving the outdoor fan 148.

The fan check routine of FIG. 8 is entered at 550, after the compressormotor is started. Decision box 552 establishes an equilibrium speedinterval of in the order of three-seconds by exiting the routine at 554if the run time variable T has not yet exceeded three seconds.

After the three-second equilibrium speed interval has elapsed, at 556V_(F) and V_(L) are sampled, and in decision box 558 the V_(F) /V_(L)ratio is tested against the fan motor lock rotor ratio which, forexample, again is 0.5. If the ratio V_(F) /V_(L) is less than 0.5, thenthe answer in decision box 558 is "yes", and the compressor is protectedby shutting down the entire system in box 560. The program then remainsin an endless loop comprising decision box 562 and branch 563 until suchtime as the reset switch (not shown) is actuated.

Returning to decision box 558, if it is determined that the fan motorhas started, then decision box 564 is entered to establish an airflowstabilization interval of in the order of ten seconds. As in the case ofthe thirty-second stabilization interval established by decision box 526in FIG. 7, in FIG. 8 the V_(F) /V_(L) ratio sampled in box 556 is storedin box 566 as reference ratio R_(F0), thus updating variable R_(F0) eachtime through the fan check routine entered at 550. The final updateoccurs at T=10 seconds, and value of R_(F0) at T=10 seconds establishesthe reference ratio R_(F0) employed thereafter.

As is discussed hereinabove, the reference ratio R_(F0) corresponds tosubstantially maximum fan motor loading when the outdoor evaporator isfree of blockage by frost.

On passes through the FIG. 8 fan check routine following the ten-secondairflow stabilization interval, in decision box 568 the then-prevailingV_(F) /V_(L) ratio is sampled and compared to a low-load threshold ratioestablished, for example, at 1.08 times the stored motor reference ratioR_(F0).

If, as a result of frost blockage, load on the evaporator fan motor hasdecreased, the V_(F) /V_(L) ratio exceeds 1.08 times the stored motorreference ratio R_(F0), and the answer in decision box 568 is "yes". Adefrosting operation is then initiated by entering box 570. In theparticular procedure of FIG. 9, defrost is effected simply byde-energizing the compressor motor for a fixed time interval, which inthis example is fifteen minutes (900 seconds) established by decisionbox 572 and branch 574.

It is not necessary that the test of box 568 be performed continuously(on every pass through the FIG. 8 fan check routine). It is generallysufficient to perform this test periodically every 120 seconds or so.

FIG. 9 is a flow chart of a fan defrost alogrithm which may be employedas an alternative to the fixed fifteen-minute defrost of FIG. 8(decision box 572) where it is desired to more accurately control thetermination of a defrosting operation, rather than relying upon a fixedtime interval. The FIG. 9 algorithm is typically applied to thedefrosting operation of the outdoor heat exchanger of a heat pumpsystem.

In FIG. 9, the outdoor heat exchanger defrosting operation is entered at600, with the time variable T initialized to zero at the beginning of adefrosting operation. Decision box 602 and routine exit 604 effect adelay of five minutes, for example, before further action is taken. Whenfive minutes have elapsed, the answer in decision box 602 is "yes", thetime variable T is reset to zero in box 606. The outdoor fan is thenmomentarily started at box 608, and allowed to come up to speed indecision box 610 which exits to 612 until a three-second interval haselapsed.

When the three-second interval has elapsed, the fan is assumed up tospeed, and the answer in decision box 610 is "yes". At that point, indecision box 614, outdoor fan loading is checked to determine whetherfan motor load has increased to a point where airflow is substantiallynormal, indicating that ice has melted.

More particularly, in decision box 614, the then-prevailing V_(F) /V_(L)ratio is compared to a factor of 1.02 times the reference ratio R_(F0)stored in FIG. 8, box 556. If the answer to the "less than" comparisonof decision box 614 is "yes", then it is assumed that the evaporator isfree of frost, and the defrosting operation is terminated at box 616.

If the answer in decision box 614 is "no", then the fan motor isde-energized in box 618, and program execution returns to box 602 tobegin another five-minute defrosting interval before the fan is againmomentarily started to check for frost blockage.

Referring finally to FIG. 10, depicted is another flow chart which maybe employed in combination with the flow charts of FIGS. 7 and 8 toguard against the possibility that a defrosting operation is neededimmediately upon compressor startup. Such could occur when a defrostingoperation in progress was terminated prematurely by a momentary powerinterruption, or by user action.

For purposes of FIG. 10, it is assumed that the compressor motorreference ratio R_(C0) has been stored as indicated by box 528 in FIG.7.

The compressor defrost algorithm of FIG. 10 entered at 580. A compressorloading interval is established, for example ten minutes, by decisionbox 582 which exits to 584 until the condition is satisified. After theten-minute compressor loading interval has elapsed, in box 586 thecompressor loading is compared to a normal load threshold ratioestablished as 0.95 times the reference R_(C0) to determine whethercompressor loading has built up to an expected normal load in theabsence of defrosting. If "yes", then the FIG. 10 routine exits at 588,and is not again entered until the compressor is again restarted. If theanswer in decision box 586 is "no", then a defrosting operation isinitiated at 590.

While specific embodiments of the invention have been illustrated anddescribed herein, it is realized that numerous modifications and changeswill occur to those skilled in the art. It is therefore to be understoodthat the appended claims are intended to cover all such modificationsand changes which fall within the true spirit and scope of theinvention.

What is claimed is:
 1. A self-calibrating method for controllingdefrosting of an evaporator in a refrigeration system which is cycled ONand OFF during operation and which includes a motor-driven fan formoving air past the evaporator, said method comprising:determining a fanmotor reference loading at a relatively early time during arefrigeration system ON cycle by allowing an airflow stabilizationinterval to elapse during which evaporator airflow stabilizes at a ratecorresponding to an unblocked evaporator, and then sensing and storingat least a representation of fan motor loading as the reference loading;and thereafter, during each ON cycle, at least periodically sensing arepresentation of prevailing fan motor loading, comparing the sensedloading to the reference loading, and initiating a defrosting operationif sensed loading is below a low load threshold loading established as apredetermined function of the reference loading.
 2. A method inaccordance with claim 1, wherein the airflow stabilization interval isin the order of ten seconds.
 3. A method in accordance with claim 1,wherein the evaporator fan motor is allowed to continue to run during adefrosting operation.
 4. A method in accordance with claim 1, whereinthe evaporator fan motor is de-energized during a defrosting operation.5. A method in accordance with claim 3, which comprises at leastperiodically checking the evaporator fan loading during a defrostingoperation to determine when defrosting is complete.
 6. A method inaccordance with claim 4, which comprises periodically energizing andchecking the evaporator fan loading during a defrosting operation todetermine when defrosting is complete.
 7. A self-calibrating method forcontrolling defrosting of an evaporator in a refrigeration system whichis cycled ON and OFF during operation and which includes a fan formoving air past the evaporator driven by a single phase induction motorsupplied from an AC power line and of the type including a capacitor-runwinding, said method comprising:determining, as an indicator ofevaporator airflow, a fan motor reference ratio at a relatively earlytime during a refrigeration system ON cycle by allowing an airflowstabilization interval to elapse during which evaporator airflowstabilizes at a rate corresponding to an unblocked evaporator, sensingthe ratio of capacitor-run winding voltage to line voltage, and thenstoring at least a representation of the sensed ratio as the fan motorreference ratio; and thereafter, during each ON cycle, at leastperiodically sensing the prevailing ratio of capacitor-run windingvoltage to line voltage, comparing the prevailing ratio to theevaporator fan motor reference ratio, and initiating a defrostingoperation if the prevailing ratio exceeds a low load threshold ratioestablished as a predetermined fraction in excess of the referenceratio.
 8. A method in accordance with claim 7, wherein the airflowstabilization interval is in the order of ten seconds.
 9. A method inaccordance with claim 7, wherein the low load threshold ratio isapproximately 1.08 times the fan motor reference ratio.
 10. A method inaccordance with claim 8, wherein the low load threshold ratio isapproximately 1.08 times the fan motor reference ratio.
 11. A method inaccordance with claim 7, wherein the evaporator fan motor is allowed tocontinue to run during a defrosting operation.
 12. A method inaccordance with claim 7, wherein the evaporator fan motor isde-energized during a defrosting operation.
 13. A method in accordancewith claim 11, which comprises, during a defrosting operation, at leastperiodically comparing the prevailing ratio of capacitor-run windingvoltage to line voltage to the reference ratio to determine whendefrosting is complete.
 14. A method in accordance with claim 12, whichcomprises periodically energizing the evaporator fan motor during adefrosting operation and comparing the prevailing ratio of capacitor-runwinding voltage to line voltage to the reference ratio to determine whendefrosting is complete.
 15. A self-calibrating method in accordance withclaim 7, which further comprises determining whether the fan motor hasfailed to start at the beginning of an ON cycle by:allowing a fan motorequilibrium speed interval to elapse; and then sensing the prevailingratio of capacitor-run winding voltage to line voltage, andde-energizing the fan motor and the compressor motor if the prevailingratio falls below a predetermined locked rotor ratio.
 16. A method inaccordance with claim 15, wherein the predetermined locked rotor ratiois in the order of 0.5.
 17. A method in accordance with claim 15,wherein the fan motor equilibrium speed interval is within the range oftwo seconds to ten seconds.
 18. A self-calibrating control system forcontrolling defrosting of an evaporator in a refrigeration system whichis cycled ON and OFF during operation and which includes a motor-drivenfan for moving air past the evaporator, said control system comprising:asensing element for sensing at least a representation of fan motorloading; means connected to said sensing element for determining a fanmotor reference loading at a relatively early time during arefrigeration system ON cycle by allowing an airflow stabilizationinterval to elapse during which evaporator airflow stabilizes at a ratecorresponding to an unblocked evaporator, and then storing at least arepresentation of fan motor loading as the reference; and meansconnected to said sensing element for thereafter, during each ON cycle,at least periodically comparing a representation of prevailing fan motorloading to the reference loading, and initiating a defrosting operationif prevailing loading is below a low load threshold loading establishedas a predetermined function of the reference speed.
 19. A control systemin accordance with claim 18, wherein the airflow stabilization intervalis in the order of ten seconds.
 20. A control system in accordance withclaim 18, which further comprises means for defrosting the evaporator,which means allow the evaporator fan motor to continue to run during adefrosting operation.
 21. A control system in accordance with claim 18,which comprises means for defrosting the evaporator, which meansde-energizes the evaporator fan motor during a defrosting operation. 22.A control system in accordance with claim 20, which comprises means forat least periodically checking the evaporator fan motor loading during adefrosting operation to determine when defrosting is complete.
 23. Acontrol system in accordance with claim 21, which comprises means forperiodically energizing and checking the evaporator fan motor loadingduring a defrosting operation to determine when defrosting is complete.24. A self-calibrating control system for controlling defrosting of anevaporator in a refrigeration system which is cycled ON and OFF duringoperation and which includes a fan for moving air past the evaporatordriven by a single phase induction motor supplied from an AC power lineand of the type including a capacitor-run winding, said control systemcomprising:sensing means for sensing the ratio of capacitor-run windingvoltage to AC line voltage; means connected to said sensing means fordetermining, as an indicator of evaporator airflow, a fan motorreference ratio at a relatively early time during a refrigeration systemON cycle by allowing an airflow stabilization interval to elapse duringwhich evaporator airflow stabilizes at a rate corresponding to anunblocked evaporator, and then storing at least a representation of theprevailing ratio of capacitor-run winding voltage to line voltage, ratioas the fan motor reference ratio; and means connected to said sensingmeans for thereafter, during each ON cycle, at least periodicallycomparing the prevailing ratio of capacitor-run winding voltage to linevoltage to the evaporator fan motor reference ratio, and initiating adefrosting operation if the prevailing ratio exceeds a low loadthreshold ratio established as a predetermined fraction in excess of thereference ratio.
 25. A control system in accordance with claim 24,wherein the airflow stabilization interval is in the order of tenseconds.
 26. A control system in accordance with claim 24, wherein thelow load threshold ratio is approximately 1.08 times the fan motorreference ratio.
 27. A control system in accordance with claim 25,wherein the low load threshold ratio is approximately 1.08 times the fanmotor reference ratio.
 28. A control system in accordance with claim 24,which comprises means for defrosting the evaporator, which means allowsthe evaporator fan motor to continue to run during a defrostingoperation.
 29. A control system in accordance with claim 24, whichcomprises means for defrosting the evaporator, which means de-energizesthe evaporator fan motor during a defrosting operation.
 30. A controlsystem in accordance with claim 28, which comprises means for, during adefrosting operation, at least periodically comparing the prevailingratio of capacitor-run winding voltage to line voltage to the referenceratio to determine when defrosting is complete.
 31. A control system inaccordance with claim 29, which comprises means for periodicallyenergizing the evaporator fan motor during a defrosting operation andcomparing the prevailing ratio of capacitor-run winding voltage to linevoltage to the reference ratio to determine when defrosting is complete.32. A self-calibrating control system in accordance with claim 24, whichfurther comprises means for determining whether the fan motor has failedto start at the beginning of an ON cycle by:allowing a fan motorequilibrium speed interval to elapse; and de-energizing the fan motor ifthe prevailing ratio of capacitor-run winding voltage to line voltage isbelow a predetermined locked rotor ratio.
 33. A control system inaccordance with claim 32, wherein the predetermined locked rotor ratiois in the order of 0.5.
 34. A control system in accordance with claim32, wherein the fan motor equilibrium speed interval is within the rangeof two seconds to ten seconds.